Most Local Area Networks (LANs) in use today are shared media LANs, Carrier Sense Multiple Access with Collision Detection (CSMA/CD) used on the Ethernet (802.3) bus or token-passing used on the token-ring LAN or Fiber Distributed Data Interface (FDDI). In some cases, there is a need for more bandwidth and facility to transport video, voice and data. Asynchronous Transfer Mode (ATM) is a transmission technology designed to transport these various traffic through LANs and through WANs. ATM is an emerging technology for cell relay and defines a fixed length 53-byte packet or cell which consists of a five-byte header and 48 bytes of user information or payload as defined by CCITT (now called ITU-T). ATM was originally viewed as a technology integrating all types of traffic, but it has been dedicated later on to the interconnection of LAN and powerful technical workstations.
The five-byte header, Network Node Interface (NNI) has a first field of 12 bits called Virtual Path Identifier (VPI), a second field of 16 bits called Virtual Channel Identifier (VCI): together these two fields identify the connection, the function is similar to the Logical Channel Identifier (LCI) in X.25 or the Data Link Control Identifier (DLCI) of frame relay.
The additional fields are: the Payload Type (PT), the Cell Loss Priority (CLP), and the Header Error Check (HEC). VPI and VCI will also be called hereafter VP and VC.
More details may be found in general documentation relating to ATM, and particularly IBM redbook GG24-4330: ATM, Technical Overview; and GG24-3178: Local Area Network, Concepts Products.
Numerous topologies are used for local area networks. The basic networks topologies are: Mesh topology, Star topology, Bus and tree topology and Ring topology. In a network that has a ring topology, each station is attached to its adjacent station by point-to-point links, thus forming a physical ring. Each station's adapter regenerates the signal as it retransmits a data packet that is circulating on the ring.
A popular protocol used with the ring topology is token-passing, in which access to the medium is controlled by possession of a circulating token. Only one token may exist on a ring segment at any given point in time. The token is a specific bit sequence (24 bits) circulating around the ring.
In an ATM ring environment dedicated to asynchronous data transfer, the transmission of isochronous traffic related to data, voice and video is not easy to perform because of the rule of priority. Indeed, the isochronous frames are to be sent/transmitted on the ring periodically, for instance, every 125 .mu.s. Therefore, the problem is to find a way to merge isochronous traffic and asynchronous traffic on an Asynchronous Transfer Mode ring.
The environment of the present invention is related to European patent application EP 95480102.3, "Method and Apparatus for Allowing Communication of Asynchronous Transfer Mod Cells in a Ring Network."